Arc tube including step-down plane portions in pinch seal area

Abstract

An arc tube that effectively prevents a metal halide from being deposited in a light emitting tube portion, causing the change of a light emitting color or the generation of lighting failures in an arc tube. An axial distance L 1 from a neck portion 20 C of an arc tube body formed between a light emitting tube portion 20 A and a pinch seal portion 24 B of the arc tube body to a step-down plane portion 20 Ba 2 of each pinch seal surface 20 Ba in a pinch seal portion 20 B is set to 1 mm or less. During pinch seal, consequently, a sufficient pinching pressure is applied to a electrode 26 , such as a tungsten electrode up to a portion close to a tip portion thereof. Thus, it is possible to reduce the volume of an almost wedge-shaped slit 24 a formed around the tungsten electrode 26 on an end in the axial direction of a discharge space 24 , thereby decreasing the amount of a metal halide deposited on the slit 24 a.

What is claimed is:  
     
       1. An arc tube comprising: 
       an arc tube body including a light emitting tube portion forming a discharge space, a pinch seal portion disposed on sides of the light emitting tube portion, and a neck portion disposed between the light emitting tube portion and the pinch seal portion; and  
       a pair of electrodes pinch sealed with the arc tube body at the pinch seal portion such that a tip portion of each of the pair of electrodes protrudes towards the discharge space,  
       wherein each of a pair of opposing pinch seal surfaces of the pinch seal portion include a general portion and a step-down plane portion formed to have a substantially planar shape in step-down with respect to the general portion, and  
       wherein an axial distance from the neck portion to the step-down plane portion is 1 mm or less.  
     
     
       2. The arc tube according to  claim 1 , wherein the arc tube body contains quartz glass and the pair of electrodes comprise a tungsten material. 
     
     
       3. An arc tube comprising: 
       an arc tube body including a light emitting tube portion forming a discharge space, a pinch seal portion diposed on sides of the light emitting tube portion, and a neck portion disposed between the light emitting tube portion and the pinch seal portion; and  
       a pair of electrodes pinch sealed with the arc tube body at the pinch seal portion such that a tip portion of each of the pair of electrodes protrudes towards the discharge space,  
       wherein an axial distance measured in a direction going away from the light emitting tube portion from the neck portion to a tip of a substantially wedge-shaped slit formed between the arc tube body and at least one of the pair of electrodes is 0.5 mm or less.  
     
     
       4. The arc tube according to  claim 3 , wherein the arc tube body contains quartz glass and the pair of electrodes comprise a tungsten material.